7. Lab-pH Meter

What is the principle of electrometric pH measurement?

Determination of the activity of hydrogen ions by potentiometric measurement using a glass pH indicating electrode.

What does a pH electrode consist of?

A silver wire coated with AgCl, immersed in a 0.1 mmol/L HCl solution, placed in a glass membrane sensitive to H+.

What causes a potential difference in a pH electrode?

Movement of H+ near the tip of the electrode when immersed in a test solution.

What is a combination pH electrode?

An electrode that contains both a glass pH indicating electrode and a built-in reference electrode.

What is the most common reference electrode?

The calomel electrode.

What is the main component of a calomel electrode?

A paste of mercurous chloride in contact with metallic mercury in a potassium chloride solution.

What does a reference electrode maintain?

A stable electrical potential as long as the electrolyte concentration and temperature remain constant.

What is the disadvantage of using a calomel electrode?

It is slow to reach a new stable voltage after temperature changes and unstable above 80°C.

What type of reference electrode can be used at high temperatures?

Ag/AgCl reference electrode.

What must be ensured during the setup of the Accumet AR10 pH Meter?

Set Mode to pH, % Slope to 100%, and Temperature to the buffer solution temperature.

What is the purpose of the Automatic Temperature Compensation (ATC) Probe?

To ensure accurate pH measurements by adjusting for temperature changes.

What should be done if the ATC probe cannot be used?

Immerse it in deionized water at the same temperature as the samples.

What is necessary before using the pH electrode?

Rinse the probes thoroughly with deionized water and blot dry.

What is the calibration procedure for the pH electrode?

Standardize it before each set of measurements using appropriate pH buffer solutions.

What buffers can be used for calibrating a pH electrode above a pH of 7.00?

4.0 and 7.0 buffers, if verified by repeated samples with 7.0 and 10.0 buffers.

What indicates the accuracy of pH measurements during calibration?

Control samples with low ionic strength HCl and certified buffer solutions.

What is the procedure for rinsing sample containers?

Rinse three times with deionized water, then once with a small amount of sample.

How long should the pH meter be allowed to stabilize?

5 minutes or until it stabilizes to the nearest 0.1 pH unit.

What is the final step for instrument shutdown?

Store electrodes properly and turn the pH Meter Mode to Standby.

What must be done to prepare the pH electrode prior to measurements?

The fill hole must be open when taking measurements.

What should you do if the electrode shows an 'ELECTRODE ERROR' message?

Inform your lab instructor immediately.

What should be done to ensure the fill hole of the pH electrode is ready?

Rotate the cap to the left to open the fill hole and add KCl if needed.

What type of solution is required for a stable voltage generation in a calomel electrode?

An electrolyte solution of potassium chloride.

What is the function of the fiber or ceramic plug in a reference electrode?

It allows a small flow of electrolyte filling solution.

Why is sodium considered an interfering cation in pH determinations?

High concentrations of sodium can affect the accuracy of pH measurements.

What is one common method for ensuring the glass pH electrode's longevity?

Store the electrode in saturated potassium chloride solution.

What is the consequence of immersing the pH electrode in deionized water?

It can lead to a slow and sluggish response.

What type of buffer solutions are used for standardizing the pH electrode?

pH 4.0, pH 7.0, and pH 10.0 buffers.

What happens if control results are not within specified limits?

Troubleshooting must be performed before analyzing any samples.

What does it mean when the pH meter reads 'GOOD ELECTRODE'?

The electrode's performance is acceptable and within the 90-102% slope range.

What should be done after completing analysis with the pH meter?

Rinse and dry the probes, then store them appropriately.

What must not happen for pH electrode operation to be accurate?

The pH electrodes must not be exposed to high concentrations of interfering cations.

Explain the term 'electromotive force' in relation to pH measurement.

The force generated by ion-exchange process at the glass surface of a pH electrode.

When calibrating, what two buffer solutions are typically used for pH measurement?

pH 4 and pH 10 buffers.

What actions are required during the calibration of the pH meter?

Immerse electrode in buffer, adjust standardization, wait for stabilization.

What is the importance of rinsing the sample container?

To prevent contamination from residual substances.

What must be ensured regarding the both reference and indicator electrodes?

They must be in contact with the sample to complete electrical connection.

What procedure must precede pH meter use to avoid errors?

Calibration or standardization must be completed prior to measurements.

What must be done if the electrolyte level in the electrode is low?

Add saturated KCl to bring the level above 1/4 inch below the cap.

What is the effect of failing to standardize the pH meter?

It could lead to inaccurate measurements and the display of an error message.

What should the user do if the electrode is outside the acceptable range?

Inform lab instructor and troubleshoot the electrode.

What database or manual is referenced in the document for further guidance?

The accumet® Basic AB15/15+ User Manual.

What is indicated by a stable pH meter reading?

The pH measurement is reliable and has stabilized.

What is the chemical formula for mercurous chloride used in calomel electrodes?

Hg/Hg2Cl2.